Presently, motorized societies are facing many problems that continue to worsen, such as the deterioration of living environs caused by traffic accidents, traffic congestion, hazardous substances, noise, etc. Intelligent transport systems (ITS) are one way to solve these problems. Vehicle Information and Communication Systems (VICS), electronic toll collection (ETC), etc. are also being put into use.
Amid such circumstances, inter-vehicle communication systems that perform wireless communication between vehicles have attracted attention, particularly in terms of safety and peace of mind. Rather than centralized management control of terminals performed by a cellular system base station, autonomous decentralized control by each vehicle (cellular terminal) performing communication is demanded of such inter-vehicle communication systems.
Thus, as a conventional control scheme for inter-vehicle communication, a Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) algorithm exists (for example, refer to Japanese Laid-Open Patent Publication No. 2008-92197). Under the CSMA/CA algorithm, vehicles transmit data after a confirmation (carrier sense) of the availability of a communication channel (propagation channel) for a given period of time.
Under the conventional technique above, however, vehicles separated by a distance that does not enable carrier sensing may simultaneously transmit data, whereby the transmitted data collides (packet collision), i.e., hidden node problem. If data collision occurs, the data cannot be decoded properly on the receiving side, arising in a problem of data loss.
Further, although Request to Send/Clear to Send (RTS/CTS) is one measure to address the problem of hidden nodes, if the size of the data to be transmitted is relatively small, as with inter-vehicle communication, use of RTS/CTS results in increased overhead. Thus, a problem of reduced communication efficiency arises.